This invention relates to semiconductor strain sensors.
As is well known, silicon has piezoresistive characteristics which vary with respect to the crystal planes chosen in which to measure such characteristics. This piezoresistive property of silicon allows fabrication of silicon stress sensitive sensors. A thin silicon disk with its major surface substantially parallel to crystal planes of a selected orientation and having sensing elements diffused through this surface serves as a diaphragm to receive applied forces. The strain occurring in the silicon in conjunction with the stress due to these applied forces results in resistance changes in the diffused sensing elements because of the piezoresistive characteristics of the silicon disk, i.e. diaphragm.
Properly constraining such a silicon diaphragm at its periphery is very important for proper operation of the stress sensor. Unsatisfactory constraints lead to error stresses being introduced both by poor bonding and by differences between the thermal coefficients of expansion of the diaphragm and the constraint. Such unsatisfactory constraints will limit the accuracy of the stress sensor.
It has previously been suggested that a satisfactory constraint for a silicon diaphragm would be one constructed of the same or a sufficiently similar material. This is usually accomplished by preparing the diaphragm and the constraint as two separate silicon components to be later joined together to form the stress sensor. Such methods require added handling and therefore expense. It also is often times difficult to obtain proper relationships concerning the relative location between the diffused resistors and the constrained periphery of the silicon diaphragm. Further, in some instances the diaphragm-constraint interface does not form properly. Therefore, an improved method of forming a constraint for a silicon diaphragm, to be used as a stress sensor, in a batch process is desirable.